platform/x86: alienware-wmi: fix format string overflow warning

platform/x86: alienware-wmi: fix format string overflow warning

gcc points out a possible format string overflow for a large value of 'zone':

drivers/platform/x86/alienware-wmi.c: In function 'alienware_wmi_init':
drivers/platform/x86/alienware-wmi.c:461:24: error: '%02X' directive writing between 2 and 8 bytes into a region of size 6 [-Werror=format-overflow=]
   sprintf(buffer, "zone%02X", i);
                        ^~~~
drivers/platform/x86/alienware-wmi.c:461:19: note: directive argument in the range [0, 2147483646]
   sprintf(buffer, "zone%02X", i);
                   ^~~~~~~~~~
drivers/platform/x86/alienware-wmi.c:461:3: note: 'sprintf' output between 7 and 13 bytes into a destination of size 10

This replaces the 'int' variable with an 'u8' to make sure
it always fits, renaming the variable to 'zone' for clarity.

Unfortunately, gcc-7.1.1 still warns about it with that change, which
seems to be unintended by the gcc developers. I have opened a bug
against gcc with a reduced test case. As a workaround, I also
change the format string to use "%02hhX", which shuts up the
warning in that version.

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81483
Link: https://patchwork.ozlabs.org/patch/788415/
Suggested-by: Andy Shevchenko <andy@infradead.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
[andy: added empty lines after u8 zone; definitions]
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>